Adds the Offerings module and retargets the CI/CD pipeline to Api.SlpSoftware

Implements Unit 2 "Offerings" (backend module, admin CRUD UI with
drag-and-drop reordering, public GET /api/v1/offerings endpoint) and
executes the feature's D-15 CI/CD cutover, switching the deploy
pipeline's build/publish target from SlpModularCms.Api to
SlpModularCms.Api.SlpSoftware.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FWyStNL2ZsjrS7FLd7xvvN

aidlc-docs/features/slpsoftware-api/construction/slpsoftware-client-setup/infrastructure-design/deployment-architecture.md

@@ -3,6 +3,7 @@
 **Status note**: this diagram documents the **target state after** the Operations-phase cutover (D-15) — it is reference/planning context for Code Generation, not something this unit deploys itself. No infrastructure changes happen as part of this Construction stage.
 
 ```mermaid
+%%{init: {'themeVariables': {'primaryTextColor':'#000000','textColor':'#000000','tertiaryTextColor':'#000000'}}}%%
 graph TD
     visitor["Site Visitor / CMS Administrator browser"]
     proxy["Proxy Pi<br/>nginx + TLS (certbot)"]
@@ -40,6 +41,7 @@ Text alternative: a visitor's browser reaches the proxy Pi over HTTPS, which for
 ## Local Development (Current Scope of This Unit)
 
 ```mermaid
+%%{init: {'themeVariables': {'primaryTextColor':'#000000','textColor':'#000000','tertiaryTextColor':'#000000'}}}%%
 graph LR
     dev["Developer machine"]
     api["SlpModularCms.Api<br/>(existing dev host)"]